An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent (EL) unit sandwiched between the anode and the cathode. The organic EL unit includes at least a hole-transporting layer (HTL), a light-emitting layer (LEL), and an electron-transporting layer (ETL). OLEDs are attractive because of their low drive voltage, high luminance, wide viewing-angle, and capability for full color displays and for other applications. Tang et al. described this multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
OLED devices are often constructed with the positive electrode or anode placed in contact with the substrate or support on which the OLED is constructed. This is known as normal or non-inverted OLED configuration. Low power consumption displays typically use an active matrix backplane where thin film transistors (TFT made of a-Si or LTPS) drive current to the OLED. In this case, the OLED stack is located at the source of the TFT, thus the anode of the OLED pixel is directly connected to the source of the driving TFT. Although this manufacturing process is much simpler, the circuit becomes dependent on the characteristics of the OLED materials. Any changes in the OLED voltage due to its aging behavior will affect both the voltage between the gate and the source (Vgs) and the current flowing through the driving TFT and OLED pixel (Ids). Alternately, if the OLED stack can be located at the drain of the drive TFT, changes in the OLED characteristics affects only the current (Ids) and not the voltage between gate and source (Vgs). But this requires an inverted OLED configuration where the cathode of the OLED is deposited first in order to connect the OLED cathode to the drain of the driving TFT. In the inverted OLED configuration, however, the deposition of organic layers must be reversed. Examples of inverted configuration are known in the art. Blochwitz et al. in U.S. Patent Application 2006/0033115, and Spindler et al. in Conference Record of the 26th International Display Research Conference, Society for Information Display, San Jose, Calif., 2006, pp. 51-54, describe some examples of inverted OLED structures. However, these devices have not performed very well due to poor or inadequate injection of holes and electrons at the anode and cathode interfaces, respectively. In particular, voltage rise over time can be very large in inverted structures. This produces difficulties in achieving desired luminance.
U.S. Pat. No. 6,436,559 (Ueno et al.), disclose the use of certain electron-deficient organic materials in a standard structure OLED device, that is, the anode is adjacent to the substrate. The preferred embodiment and example use the electron-deficient material in a layer adjacent the cathode.
U.S. Pat. No. 6,720,573 (Son et al.), disclose the use of certain electron-deficient organic materials in a standard structure OLED device. In this case, the material is used in a layer adjacent the anode.
Both Ueno et al. and Son et al., allege improved lifetime and lower initial operating voltage relative to devices not having such layers.
The present invention calls for a combination of two electron-accepting layers using electron-deficient organic materials in an inverted structure, that is, the cathode is adjacent to the substrate. One electron-accepting layer is provided between the hole-transporting layer and the anode whereas the other is provided between the electron-transporting layer and the cathode. Applicant has found that this combination of features provides the unexpected advantage of reduced voltage rise over the operating life of the device. There is no teaching or suggestion in Ueno et al. or Son et al., alone or in combination, for the presently claimed invention. Neither patent recognized that our claimed combination was necessary to provide this unexpected result of reduced voltage rise over time.